Agricultural tractors have traditionally been unsprung. From their earliest beginnings in the late 1800's they have been supported on fixed axles extending from the chassis.
Originally tractors were used as stationary engines. Located in a fixed position in a field, farmers would gather crops to be threshed and bring them in wagon loads to the tractor and a belt-driven threshing machine. In these early days the ability to move fast was not important.
Tractors were gradually modified to tow implements such as plows, rakes, harrows, planters, and manure spreaders through agricultural fields. These mobile tractors did not need a great deal of speed since they replaced horses or oxen and needed only enough power to tow implements at horse or ox speed.
As time passed, engineers designed ever larger and stronger implements. To tow these implements, tractors were also made stronger and larger, with ten to fifty times the horsepower of the early tractors.
Eventually, agricultural tractors were capable of towing implements at higher speeds through agricultural fields. To accommodate these greater speeds, manufacturers began to develop front suspensions with springing and shock absorbing capability. These front suspensions were configured to pivot, permitting the front wheels of the tractor to keep a good grip on the ground as the terrain changed. As of today, however, no major manufacturer of tractors sells a commercially accepted agricultural tractor with a sprung rear suspension.
A primary reason that tractors with sprung rear suspensions have not been manufactured is due to the reaction forces that arise when a load is placed on the tractor. Traditional agricultural tractors have large rear wheels, typically on the order of approximately 1 to 2.2 meters in diameter. The large rear wheels apply high force to the ground, especially when a ground-engaging implement is ripping furrows through the ground 2 to 18 inches deep. The ground, in turn, applies an equally high (but in the opposite direction) reaction force on the frame of the tractor, and the reaction force can generate a moment great enough to literally lift the front wheels of a tractor without a rear suspension a meter or more off of the ground.
The existence of a moment large enough to lift the front wheels is best illustrated with reference to FIG. 8, which schematically shows a tractor 700 without a front or rear suspension towing an implement 148. An implement, resultant-force vector 402 is applied to the implement by the ground as the implement is pulled through the ground by the tractor 700. Implement force vector 402 can be broken down into two force vectors 404, 406 that represent the horizontal force (vector 404) acting to drag on the implement during forward motion, and the vertical force (vector 406) that pulls downward on the implement.
The implement is rigidly coupled to the tractor typically through a three-point hitch. The three-point hitch couples the implement to the tractor frame via a lower point A and an upper point B. The implement force vector 402 applies draft forces on the tractor that can be separated into horizontal and vertical forces FAx and FAy acting through the lower link 902 (i.e., at point A) and horizontal and vertical forces FBx and FBy acting through the upper link 904 (i.e., at point B). As one of ordinary skill will appreciate, the relative magnitudes of the component draft forces FAx, FAy, FBx and FBy depend upon the geometry of the three-point pitch.
Other forces acting on the tractor 700 include weight (depicted in the drawing as mg), which acts on the center of gravity CG. In response to the weight, the ground applies forces Ff and Fr to the tractor through the front and rear axles, respectively.
There are torques shown in FIG. 8 as well. Drive torque TD is the torque applied by the engine (not shown in FIG. 8) to the axle (also not shown) to drive the rear wheels. When the tractor is being driven forward, the drive torque is clockwise. The rear wheels, as they are being driven, apply a force on the ground, and the ground, in turn, applies an equal and opposite traction force FTr on the wheels that is applied to the tractor frame. The traction force of course is responsible for forward movement of the tractor.
Drive torque TD also generates a reaction torque (that is, traction torque TTr) that acts on the frame of the tractor. The traction torque is proportional to the traction force FTr and is counterclockwise.
The forces and torques generate moments about a point on the tractor that tend to rotate the tractor about that point. For convenience, the point will be called the center of pitch Cp. Its location depends upon a number factors one of ordinary skill will appreciate. While the forces and torques may generate moments that cancel each other out to some extent, the net effect of all of the moments is to generate a counterclockwise moment Mp about the center of pitch when the implement force vector 402 increases. The implement force vector increases when the implement 148 hits a stone, compacted soil, or some other such condition. As previously mentioned, the increased implement force vector can be large enough to cause a moment MP about the center of pitch that is itself large enough to lift the front tires and increase the load on the rear tires.
If the rear wheels were suspended on the frame rather than being fixed, the moment MP will not at first lift the front wheels, but it will tend to cause the rear suspension to squat. Such squatting can be disconcerting to the operator and can also wreak havoc on implement depth-control systems, which typically require a constant relationship between the tractor-frame and implement-frame heights.
One of ordinary skill will appreciate that some suspension configurations will cause the tractor to rotate clockwise (rather than counterclockwise, as has been described) when the tractor is subjected to increased loads. However, for the purposes of this discussion, we will consider the more intuitive case where the tractor rotates counterclockwise in response to increased loads. Nevertheless, the basic principles (and the problems with conventional systems) described herein are the same. Moreover, the principle of operation of the preferred embodiments (which will be described below) is the same regardless of whether the suspension tends to squat or sit up.
The suspension arrangement of the present invention generates a reaction torque on the vehicle to reduce the moment MP about the center of pitch. In other words, when the tractor pulls harder on its implement, the suspension in accordance with the present invention generates an increased counteracting, or reaction, force that matches or is proportional to the increased, horizontal force vector 404. Similarly, when the tractor pulls more gently on its implement, the suspension in accordance with the present invention generates a decreased force that matches the decreased horizontal force vector 404.
The parent application describes a tractor that would solve many of these problems. The embodiments shown in the parent application have trailing swing arms that are pivotally coupled to the chassis forward of the rear axles. The particular arrangement of these links provide certain benefits, such as resistance to suspension squatting or sitting up when the load is applied and then released. The trailing link arrangement, however, does make the suspension arms more susceptible to twisting. The arms are relatively long, and (in a preferred embodiment) are subject to flexure when the rear wheels are extremely loaded.
This flexure is caused by the wheel forces acting upward on the ends of the axles twisting the suspension arm, the forces being offset from the central line of the arms, because of the large wheel span or the use of rear duals tires or both. The fact that any twisting occurs at all is due in large part to the significant loads applied to the tractor by the implements that it pulls. As explained above, implements can exert extreme loads, both backwards and downwards, on the rear of a tractor. These loads are well beyond that experienced by a typical automobile or truck.
Tractors must be engineered to handle these loads in order to have sufficient power to tow ground engaging implements through an agricultural field.
Another reason the torsion arms experience large twisting loads is due to the direction of the loads. Implement supply loads that are directed both backward and downward, and the downward component of the force asked directly on the trailing link suspension arms to twist them.
Given the extreme loads that an implement applies to the rear of the tractor, if even 25% of this force is a downward components of the force, the compress of loads on the suspension of the tractor can be extreme, on the order of over a thousand pounds of downforce.
Another reason the suspension arm is subject to twisting is due to the less than optimal location of the wheels. Unlike an automobile, which as closely spaced wheels directly adjacent to the suspension arm, the rear wheels of a typical tractor are spaced 2 to 4 meters apart at the rear of the vehicle.
Tractor wheels are mounted towards the end of a driven axle that may be some 0.5, 1.0, 1.5, or even 2.0 meters from its associated suspension arm. The reactive ground force acting through the wheels on the axle that extends from the suspension arm, has a substantial moment arm and therefore generates a significant torque or torsional moment at the suspension arm.
One seemingly plausible solution to the problem would be to make the suspension arms extremely strong, such that they resist any torsional moments or torques that is applied to them by their respective axles. Unfortunately, suspension components should be as light in weight as possible to reduce the unsprung mass of the vehicle.
What is needed, therefore, is a system that stiffens or strengthens a rear suspension for tractor to resist the twisting forces applied to the suspension arm. What is also needed is a system for stiffening or strengthening a tractor suspension that does not require significant additional mass.
It is an object of this invention to provide a tractor and tractor suspension that has these benefits.